The invention relates to a novel process for conditioning isocyanates.
In the production and processing of isocyanates, organophosphorus aids and additives are occasionally used. These organophosphorus aids and additives as used or in the form of secondary products, can lead to problems in the later processing of the isocyanates.
For instance, in the polyisocyanates containing uretdione groups accessible by the teaching of DE-A 1670720, for example, occasionally the oxides of the trialkylphosphines used as catalysts occur which can also be formed from other P-containing intermediates products, in particular the trialkyldihalophosphoranes. All of these P-containing species can accumulate in the process in the case of repeated use of the monomer separated off in the process typically by distillation, and this leads to problems on reuse of the monomeric isocyanate. A simple removal by distillation fails because of the catalytic activity in particular of the trialkylphosphine oxides towards isocyanates which leads to unacceptably high losses of yield having to be accepted.
There has been no lack of attempts to remove the interfering organophosphorus components in a suitable manner (i.e. conditioning of isocyanates). In these various attempts, however, it has proved that either no change of these interfering organophosphorus components occurs, or else that the isocyanate to be conditioned reacts with the conditioning agent to an unacceptable extent.
Conversion of organophosphorus compounds of the abovementioned type into phosphine sulfides by means of P4S10 is described, at least for some of such compounds, for example in Houben-Weil, Vol. XII/1, pp. 168 ff and literature cited therein. However, trialkylphosphine oxides are only suitable for conversion into the corresponding trialkylphosphine sulfides by means of P4S10 in individual cases, according to Houben-Weil, Vol. E2 (Organic Phosphorous Compounds II), pages 87 ff and literature cited therein. Where and to what extent these conversion products are inert towards NCO groups and the suitability for conversion and removal of trialkylphosphine oxides from isocyanates is not apparent from the prior art.
The object of the invention was to provide a suitable process for eliminating trialkylphosphine oxides and trialkyldihalophosphoranes from isocyanates or mixtures thereof which enables the isocyanate to be conditioned to be recovered in high yield and high purity without having the problem of interfering side reactions with the conditioning agent or its secondary products.
Surprisingly, it has now been found that treating trialkylphosphine oxide-containing isocyanates and mixtures which may be contaminated with additionally P (phosphorus) containing impurities such as, for example, trialkyldihalophosphoranes, with P4S10 as a conditioning agent achieves the abovementioned object.